1. Field
The disclosed concept pertains generally to transfer switches and, more particularly, to automatic transfer switches including a first automatic transfer switch and a second bypass switch.
2. Background Information
Transfer switches are employed to protect critical electrical loads against loss of power from a first power source backed up by a second power source. The transfer switch is electrically connected to both of the power sources and supplies a number of loads with power from one of the two power sources. In the event that power is lost from the first power source, the transfer switch transfers the load to the second power source. This transfer can be manual or automatic. After the first power source is restored, the load is transferred back to the first power source.
In an automatic transfer switch (ATS), a controller or other intelligence of the ATS initiates the transfer when the first power source fails or falls below, or rises above, a predetermined voltage. If the second power source is a standby generator, then the ATS initiates generator startup and transfers to the second power source when sufficient generator voltage is available. When the first power source is restored, the ATS automatically transfers back to the first power source and initiates generator shutdown. In the event that the first power source fails and the second power source does not appear, then the ATS remains electrically connected to the first power source until the second power source does appear. Conversely, if electrically connected to the second power source and the second power source fails while the first power source is still unavailable, then the ATS remains electrically connected to the second power source.
An ATS automatically performs the transfer functions and can include, for example and without limitation, a power contactor or two circuit breakers to implement the ATS operations, and a controller (or other intelligence or supervisory circuit) to constantly monitor the condition of the power sources and provide automatic control of the switch and related circuit operation.
Certain systems and installations (e.g., without limitation, data centers; hospitals; water treatment plants; other critical processes that need the constant supply of power to a system load) employ a power system structured to provide an uninterruptable power supply (UPS). UPSs are well known. The first or primary power source is usually an electric utility or the public power grid, and the second or secondary power source is usually a generator. Alternatively, there could be two generators used instead of the utility or public power grid, or there could be two utilities. Basically, any suitable power sources that need to be switched with a load output can be employed.
A bypass/isolation switch is an assembly used with a transfer switch to select an available power source to feed load circuits, and to electrically isolate the transfer switch for inspection and maintenance.
An ATS bypass assembly has two separate switch assemblies or switches (e.g., an ATS switch and a bypass switch) that can transfer power to the system load. Each of the two switches has two positions to power the load from the first and second power sources. For example and without limitation, each of the two switches can be provided by a three-position contactor or two circuit breakers. For example, a three-position switch has the capability of having a first position electrically connected to the first power source, a second position electrically connected to the second power source, and a third neutral, open or trip position between the first and second positions. Both the ATS switch and the bypass switch are normally electrically connected to the system load. Furthermore, each of the ATS and bypass switches can be structured, for example, for use with single-phase or three-phase systems. These switches can selectively engage one power source at a time unless a closed transition type scenario is applied.
Typically, the entire ATS bypass assembly is called a “switch” and includes two sub-switches: the ATS sub-switch and the bypass sub-switch. The ATS sub-switch (hereinafter, “ATS switch”) is the main sub-switch used to transfer power. The bypass sub-switch (hereinafter, “bypass switch”) is typically used when the ATS switch is undergoing maintenance. The ATS switch is structured to easily be removed for maintenance. The bypass switch is usually fixed within the enclosure of the ATS bypass assembly, although a removable type of bypass switch is known. See, for example, U.S. Pat. No. 7,973,253.
If the bypass switch is a fixed type, the main difference between the ATS switch and the bypass switch (for contactors) used in the ATS bypass switch is that the bypass contactors are fixed with no truck or rail enabling draw-out. Otherwise, with a dual draw-out version of the bypass switch, both contactors can be removed (e.g., by trucks) with the power from the first and second power sources turned off.
For example and without limitation, in the ATS bypass switch, the bottom position of the assembly includes the ATS switch and the top position includes the bypass switch, although any suitable positioning of the two switches is possible.
During the life of the ATS bypass switch, the ATS switch contactors or circuit breakers usually do nearly all of the current transfer for the system loads. The ATS switching device is mounted with safety interlocks, in a “truck” or “slider rail” mechanism, thereby allowing the ATS switching device to be “drawn-out” for service, maintenance and/or replacement.
An ATS draw-out switching device has, for example, three positions with a compartment door closed (e.g., locked in; isolated; removed). See, for example, U.S. Pat. No. 7,973,253. Locked in is the normal operating position. Removed is the position for service, maintenance and/or replacement. In the isolated position, the ATS draw-out switching device is fully electrically disconnected (both the primary and the secondary connectors) from the ATS bypass switch, and is ready for removal. For example, the ATS draw-out switching device includes both primary disconnects (for the first and second power sources, and the load) and secondary disconnects (for control and feedback). The operating mechanism is electrically operated and can also have a mechanical operation if needed in an emergency. When withdrawn (removed), the ATS draw-out switching device can be inspected, tested and suitably maintained.
With the ATS draw-out switching device in the isolated or removed position, the bypass switch can be used to switch the two power sources. Known bypass switches of known ATS bypass switches are either switched manually (with electrical power on one of the two power sources), or switched manually with a suitable tool (with electrical power removed from both of the two power sources).
For example, known bypass switches (e.g., employing contactors or circuit breakers) of known ATS bypass switches operate only manually. Since manual operation is required whenever the ATS switch is removed, personnel (e.g., maintenance personnel) must remain located near the ATS bypass switch if power from the currently employed power source goes off and manual switching of the bypass switch to the other power source is required.
There is room for improvement in ATS bypass switches.